Official Di2 hydraulic discs are soon in the offing-
http://road.cc/content/news/76756-revealed-shimano-di2-hydraulic-disc-brakes-are-coming
I'm with Ian on the whole disc thing. Some of my riding acquaintances are of the views that they don't want them- why for a summer bike/ugly/callipers work well enough/heavy....
Cable discs are allegedly more bother than hydros- well, in a year and a bit and 6,300 or so miles so far on the Trek, no bother at all. Discs don't care if your rims are covered in crud, or it's raining, or they're aluminium or carbon. They'll just work. And for an (expensive) summer bike- say you'd like a nice flash set of carbon clinchers. You go on a holiday to the Alps or the Pyrenees. Which braking solution is far safer for a 50 mph descent? The one that is far less likely to cause your wheels to delaminate or your tyres to burst, I'd say. For that reason alone, I'd say discs will be getting a lot more popular for road bikes.
I'm kind of surprised that Trek haven't made any moves to launch road bikes (rather than the Portland and its Crossrip successor) with discs yet - they're taking a wait-and-see approach (officially, on the technology and the demand, though I'm sure they're paying attention to how Spesh do with the Roubaix & Secteur disc versions). Domane disc would be my kind of bike. Unless I had the £££££ for a Parlee Z-Zero....